High speed serial communications implemented on printed circuit boards (PCBs) are becoming increasingly popular for high bandwidth data transfer. PCBs typically are made up of multiple layers or planes including power planes, ground planes, and signal planes. Vias are employed to route traces for transmitting signals from the top layer of a PCB to a lower layer of a PCB, or to route traces from one layer to another layer within the PCB. Vias are conductors that connect traces from one layer in a PCB to traces in another layer in a PCB. When vias pass through a power or ground plane, the conducting material around the via on the power or ground plane is removed to prevent a short between the via and the power or ground plane. The area that is removed creates a void called an anti-pad.
A stray or parasitic capacitance is formed between the via barrel and the conductive material of the power or ground plane near the via barrel. This stray capacitance is inversely proportional to the size and surface area of the anti-pad. In other words, as the size and surface area of the anti-pad increases, the stray capacitance decreases, and as the size and surface area of the anti-pad decreases, the stray capacitance increases. For low speed signals, such as signals less than approximately 2 GHz, the stray capacitance typically does not have an appreciable effect on signal integrity. As signal speeds increase to greater than approximately 2 GHz, however, the stray capacitance has an increasingly more significant effect on signal integrity. Therefore, it is desirable to reduce the stray capacitance when transmitting high speed signals through vias on PCBs.
Various approaches have been proposed to reduce the stray capacitance. One approach is to increase the size of the anti-pad. This approach, however, can result in non-planarity issues of layers or planes within PCBs. Non-planarity typically occurs during the manufacturing of a PCB when dielectric material settles into the voided area of the anti-pad. The settled dielectric material causes dips on the board surface around the via, thereby reducing the planarity of the board surface. As the anti-pad size is increased to reduce stray capacitance, the non-planarity of the PCB also increases due to the increased voided anti-pad area in which dielectric material settles.
Increasing the size of the anti-pad may also lead to a choking off of power or ground planes in areas where there are many vias situated close together, such as where an integrated circuit is mounted to a PCB. Power or ground planes can be choked off when too much conductive material is removed from the power or ground planes to form the anti-pads of increased size. The choking off of power or ground planes typically prevents signal traces running between the vias on adjacent signal layers from having a good power or ground reference on the power or ground layer where the anti-pads are formed.
Other approaches reduce the stray capacitance by removing or not fabricating unused portions of the via. The stray capacitance is reduced by decreasing the number of power or ground planes that a via must pass through. One such approach uses blind vias or buried vias, which are vias that do not pass completely through a PCB. The disadvantages of this approach are that blind or buried vias may increase fabrication costs and may not support known soldering techniques, such as pin-in-hole soldering techniques. Another approach utilizes a drill to counter bore and remove unused portions of a via. A disadvantage of this approach is an increase in fabrication costs.
Another approach takes an entirely different avenue to dealing with stray capacitance. Rather than attempting to minimize the stray capacitance, this approach actually increases the capacitance to set values. The set values of capacitance are used in an attempt to optimize the frequency response characteristics of a via or as part of a filter for signals transmitted through the via. The disadvantage of this approach is that different via and anti-pad designs are required based upon the signal that will be transmitted through the via.
High speed signals on a PCB commonly originate within an integrated circuit such as an application specific integrated circuit (ASIC) mounted to the PCB. The integrated circuit may be mounted to the PCB in any number of ways, including using different soldering techniques and sockets that allow the integrated circuit to be removed and remounted on the PCB. To mount integrated circuits to a PCB, the planarity of the PCB must be maintained to a tight tolerance to maintain a high integrity signal between the integrated circuit and the PCB. Non-planarity of the PCB reduces this tight tolerance and therefore the integrity of the signal.
For reasons stated above and for other reasons presented in the present specification, there is a need for a PCB that includes anti-pad designs associated with vias that will minimize capacitance, maximize board planarity, and minimize signal trace routing issues, thereby allowing high speed serial communications regardless of the environment or application.